Thanks for your query.
Generally both the testis are similar in size. Though there could be slight disparity, but never like what you mention. First rule out a hydrocele(accumulation of fluid in side the testicular sac). Then get your correct testicular volume using an orchidometer
or an ultrasound
. If you had confirmed that both the testis are still differant size, as long as testis is funtioning well, you need not be bothered.
I do hope that you have found something helpful and I will be glad to answer any further query.